As described in the above-identified '753 application, in some radio bands, such as the 217-220 MHz VHF band, as a non-limiting example, governmental licensing agencies (e.g., the Federal Communications Commission (FCC)) customarily grant primary licensees non-exclusive use of the band for a variety of communication services, such as push-to-talk voice transmission. These primary users pay for this licensed use with an expectation that they will not encounter interference by other users. The FCC also allows secondary users to access the same band and the same channels within the band on a ‘non-interfering’ or secondary basis, whereby a channel may be used by a secondary, non-licensed, user, so long as the primary user is not using that channel.
The FCC and similar agencies in foreign countries are continually looking for ways that allow expanded use of these licensed radio frequency bands, without reducing the quality of service available to the primary users. For secondary users, these bands provide a cost-free opportunity with excellent radio transmission properties for telemetry and other applications. Because secondary users must not interfere with primary users, complaints of interference from a primary user to the FCC may result in its issuing an administrative order requiring that the secondary user move to another portion of the band or leave the band entirely. Such a spectral transition is disruptive to the secondary user's service and can be expensive, especially if site visits, equipment modification, or exchange are required, in order to implement the mandated change. It will be appreciated, therefore, that there has been a need for a mechanism that allows a secondary-user to employ a licensed band on a non-interfering basis and will adapt the radio's frequency usage should new primary users appear. It should be noted that primary users always have priority over secondary users, there is no first-use channel frequency right for secondary users.
Advantageously, the invention described in the above-referenced '753 and '105 applications successfully addresses this need by means of a monitored spectral activity-based link utilization control mechanism. Briefly reviewing this link utilization control mechanism which may be used with a star-configured communication system among other configurations and topologies, such as that depicted in the reduced complexity diagram of FIG. 1, a spectral reuse transceiver installed at a master site 10 communicates with respective spectral reuse transceivers installed at a plurality of remote sites 12. Each spectral reuse transceiver operates with a selectively filtered form of frequency hopping for producing a sub-set of non-interfering radio channels also referenced herein as ‘sub-channels’. The network uses a dynamic hopping sequence based on interference measurements. A pseudo-random sequence is used to select hopping channels in the band that are not busy. If, for example, the network is using 20 hopping channels simultaneously to achieve the desired bandwidth, it will select twenty of the available hopping channels out of the available (non-busy) hopping channels and transmit in those hopping channels for a dwell period. It will then select another set of twenty available hopping channels out of the available hopping channels and use those during the next dwell period. This process continues until the continuing spectral analysis, described earlier, detects a change to the list of available hopping channels (new interference or formerly busy or blocked hopping channel becomes available). After that time, new hopping sequences are used in the network, to take into account the change in interference analysis. It should be noted here that other configuration or network topologies may be used consistent with the invention disclosed herein.
Thus the invention may be used with radio links between transceivers in other topologies, such as point-to-point, and individual links in mesh networks, without limitation, consistent herewith.
For this purpose, the master site 10 periodically initiates a clear channel assessment routine; in which the master site and each of the remote sites 12 participate, in order to compile or ‘harvest’ a list of non-interfering or ‘clear’ sub-channels such as, by way of example and not limitation, 6.25 KHz wide sub-channels, which may be used by participants of the network for conducting communication sessions that do not ostensibly interfere with any licensed user. By transmitting on only clear sub-channels, a respective site's spectral reuse transceiver is ensured that it will not interfere with any primary user of the band of interest.
Except when it is transmitting a message to the master site, each remote user site sequentially steps through and monitors a current list of clear channels that it has previously obtained from the master site, in accordance with a pseudo-random (PN) hopping sequence that is known a priori by all the users of the network, looking for a message that may be transmitted to it by the master site transceiver. During the preamble period of any message transmitted by the master site, each remote site's transceiver scans all frequency bins within a given spectrum for the presence of energy. Any bin containing energy above a prescribed threshold is marked as a non-clear channel, while the remaining channels are identified as clear channels thus available for reuse.
Whenever a remote site notices a change in its clear channel assessment, it reports this to the master site at the first opportunity. As the master site has received clear channel lists from all the remote sites, it logically combines all of the clear channel lists, to produce a composite clear channel list. This composite clear channel list is stored in the master site's transceiver and is broadcast to all of the remote sites over a prescribed one of the clear channels that is selected in accordance with a PN sequence through which clear channels are selectively used among the users of the network. When the composite clear channel list is received at a respective remote site it is stored in its transceiver.
To ensure that all communications among the users of the network are properly synchronized in terms of a composite clear channel list and the order through which the units traverse, or ‘hop’ through, the clear channel entries on that list, the master site's transceiver transmits an initialization message on an a priori established clear channel, which each of the remote units monitors. This initialization message contains the clear channel list, an identification of the preamble channel, a PN sequence tap list, and a PN seed that defines the initial channel and hopping sequence for the duration of an upcoming transmit burst. Once a remote site has received an initialization message, that site will transition to normal multiple access mode.
For further details of the architecture and operation of the spectral reuse link control mechanism disclosed in the above-referenced '753 application, the contents of which are hereby incorporated by reference, attention may be directed to that document. They will not be detailed here, in order to focus the present description on the problem of ‘FEC degradation’, whereby varying and unpredictable atmospheric or other conditions, which can give rise to multipath, and man-made noise cause errors in burst transmissions; and whereby transmissions on formerly clear channels that were recently ‘clear’ and thereby potentially available for secondary reuse sub-channels are suddenly in use for a sustained period by another primary or secondary ‘interferer’ and collide with said transceiver's transmissions. Both these and similar events undesirably degrade the performance of the receiving transceiver's FEC circuit.